[Tarantool-patches] [PATCH 1/2] replication: introduce ballot.can_be_leader

[Vladislav Shpilevoy]

On 20.07.2021 10:49, Konstantin Osipov wrote:
> * Vladislav Shpilevoy <v.shpilevoy at tarantool.org> [21/07/20 01:09]:
>>>>> Curious why did you add this feature in the first place, I mean
>>>>> "eligibility"? Each voter has to be able to become a leader,
>>>>> otherwise raft liveness guarantees are violated. Raft has
>>>>> learners, but learners neither vote nor can become leaders.
>>>>
>>>> Voters are nodes which an admin does not want to be a leader. For
>>>> instance, they are too far away physically. As voters, they might
>>>> help to elect a leader, for example, if there are just 3 nodes one
>>>> of which is a voter.
>>>>
>>>> Another application is when you specifically start 1 node as a
>>>> voter and 2 candidates. The voter might skip all the replication
>>>> data and work on a slow small machine.
>>>>
>>>> It can help to form a majority. We are planning to make this
>>>> feature even easier to use by adding dataless nodes just for
>>>> voting.
>>>>
>>>> As for Raft, it should not bring any problems. In Raft you can
>>>> say that all nodes are candidates, but some of them are so slow,
>>>> that they can never vote for themselves in time. Raft still works,
>>>> and you essentially have 'voters'.
>>>
>>> Imagine there are nodes A, B, C, D, E.
>>> A is a leader, E is a voter which can not become a leader.
>>>
>>> Imagine A's log index is 5, B = 4, C = 3, D = 2, E = 5.
>>>
>>> The majority's log index is 4, so entry 4 is committed. A dies, B
>>> is partitioned away. The cluster is stuck, because neither C nor B
>>> can get a quorum (3 votes).
>>
>> But how is it different from the real Raft? In normal Raft I can say
>> E simply is too slow to make any actions. It is just stuck or died.
>> The cluster will be stuck then, yes. Not much you can do here.
> 
> In a real raft:
> - liveness is guaranteed if quorum is present; this guarantee here
>   is not held
> - you never sacrifice safety for liveness; you never lose
>   committed entries if quorum is present; and you never lose it
>   unnoticed! here you can lose a committed entry and not notice
>   it.

Please, show me an example. The example above only shows that the
election might stop if there are issues with the quorum. And this
will happen regardless of whether I have voter role or not. In normal
Raft you can kill 3/5 nodes and nothing will work too.